Stomatal conductance is the measure of the rate of passage of carbon dioxide (CO2) entering, or water vapor (H2O) exiting, through the stomata of a leaf (in mmol m⁻² s⁻¹). You can use stomatal pore size and density or a leaf water loss experiment to estimate conductance.
The modern way is to use a leaf porometer or gas exchange chambers with infra-red analyzers :)